This invention relates to spirally wound electrochemical cell components and method and apparatus for their production, and more particularly relates to the production of geometrically uniform spiral cell packs accommodating dimensional variations in components, where at least one of the components is readily compressible.
Various techniques for spirally winding electrochemical cells into a generally cylindrical ("jelly roll") configuration are known, and include the driven mandrel type (e.g. U.S. Pat. No. 3,298,871 to Binder et al), the driven mandrel and idling pressure roll type (e.g., U.S. Pat. No. 1,269,778 to Becker and U.S. Pat. No. 3,839,088 to Hug et al). The use of a single endless belt serpentined about a series of rollers, for instance of the general type shown in U.S. Pat. No. 171,346 to Broas, has also been used in spirally wind electrochemical cell packs; a weight tensioning device is attached to one of the rollers to produce a cell pack of uniform mutual stacking pressure between the components.
While the aforementioned types of spiral winders have their benefits in particular applications, one disadvantage prevalent with each of the above types is that the resultant cross-sectional geometrical configuration of the cell pack will vary significantly with relatively small variances in component (i.e., plate and separator) thicknesses. The obvious practical problems resulting from these inconsistent geometrical cross sections is that the outside diameter of the "cylindrical" cell pack is oftentimes too large for the cylindrical container in which it is to be stuffed, or alternately is too loose to snugly fit into such container. As a result, the scrap rate for such spirally wound elements may become unbearably high, or it may become necessary to maintain very close tolerance control over incoming components, resulting in increased production costs.
Another important criteria for a spiral winder is that it provide for limited differential slip between the plate and separator components during winding. As the components are being wound upon themselves the outermost components are normally placed in tension, while the inwardly adjacent component is following a winding curvature defined by a smaller radius than the outermost component and will therefore by put either into compression or relatively less tension than the outermost component. Particularly where fragile or dimensionally unstable components are being wound, the inability of the components to slip one with respect to another may result in separator stretching or tearing, the separator folding back upon itself, and/or the plates folding back upon themselves. It is known that any of the just described phenomena can lead to premature cell failure due to internal shorting, particularly in rechargeable cells which experience plate growth during cycling.
The aforementioned problems which develop when there is improper relative slip between the components is accentuated when the separators are made of a material which is extremely fragile and difficult to handle, such as nonwoven ultra fine mats of fiber glass or other high heat of wetting material. The problem may become particularly critical when employing plates utilizing soft and pliable substrates onto which is affixed a sticky or tacky paste material which may become physically bonded to the separator strips in advance of the point of wind, and therefore preclude relative slipping during wind.
It is a primary object of the subject invention to overcome the prior mentioned problem; to provide a wound cell element having a very uniform cross-sectional geometry even though the components may vary substantially in thicknesses; to provide a wound element in which the components are in alignment with respect to each other; to provide a wound element in which the pasted surfaces of the cell plates are not marred, scratched, allowed to crack, or otherwise damaged during winding; to provide a winding process in which the separators and plates are permitted to slip relative to one another during winding while maintaining proper winding pressure; and to provide a winding apparatus to meet the aforementioned and other objects of the invention.